Python List Advanced Exercise - Reverse a list at a specific location

Last update on April 19 2025 13:03:21 (UTC/GMT +8 hours)

1. Reverse List at Specific Location

Write a Python function to reverse a list at a specific location.

Sample Solution:

Python Code:

# Define a function to reverse a portion of a list in place
def reverse_list_in_location(lst, start_pos, end_pos):
    # Use a while loop to swap elements from the start and end positions
    while start_pos < end_pos:
        # Swap elements at start_pos and end_pos using tuple unpacking
        lst[start_pos], lst[end_pos] = lst[end_pos], lst[start_pos]
        # Move the start_pos towards the end_pos and vice versa
        start_pos += 1
        end_pos -= 1
    # Return the modified list
    return lst

# Create a list of numbers
nums = [10, 20, 30, 40, 50, 60, 70, 80]

# Define the start and end positions for the first reverse operation
start_pos = 2
end_pos = 4

# Print the original list
print("Original list:")
print(nums)

# Print a message indicating which portion of the list will be reversed
print("Reverse elements of the said list between index position " + str(start_pos) + " and " + str(end_pos))

# Call the reverse_list_in_location function and print the result
print(reverse_list_in_location(nums, start_pos, end_pos))

# Define the start and end positions for the second reverse operation
start_pos = 6
end_pos = 7

# Print a message indicating which portion of the list will be reversed
print("\nOriginal list:")
print(nums)
print("Reverse elements of the said list between index position " + str(start_pos) + " and " + str(end_pos))

# Call the reverse_list_in_location function again and print the result
print(reverse_list_in_location(nums, start_pos, end_pos))

# Define the start and end positions for the third reverse operation
start_pos = 0
end_pos = 7

# Print a message indicating which portion of the list will be reversed
print("\nOriginal list:")
print(nums)
print("Reverse elements of the said list between index position " + str(start_pos) + " and " + str(end_pos))

# Call the reverse_list_in_location function once more and print the result
print(reverse_list_in_location(nums, start_pos, end_pos))

Sample Output:

Original list:
[10, 20, 30, 40, 50, 60, 70, 80]
Reverse elements of the said list between index position 2 and 4
[10, 20, 50, 40, 30, 60, 70, 80]

Original list:
[10, 20, 50, 40, 30, 60, 70, 80]
Reverse elements of the said list between index position 6 and 7
[10, 20, 50, 40, 30, 60, 80, 70]

Original list:
[10, 20, 50, 40, 30, 60, 80, 70]
Reverse elements of the said list between index position 0 and 7
[70, 80, 60, 30, 40, 50, 20, 10]

Algorithm: reverse_list_in_location(lst, start_pos, end_pos) function

Input: A list “lst” and two integers start_pos and end_pos indicate the start and end positions of the sub-list that needs to be reversed.

Output: The input list “lst” with the sub-list from start_pos to end_pos reversed.

Steps:

Initialize two variables start_pos and end_pos to the start and end positions of the sub-list.
While start_pos is less than end_pos do the following:

Swap the elements at start_pos and end_pos
Increment start_pos by 1
Decrement end_pos by 1

Return the list lst.

Flowchart:

What is the time complexity and space complexity of the following Python code?

def reverse_list_in_location(lst, start_pos, end_pos):
    while start_pos < end_pos:
        lst[start_pos], lst[end_pos] = lst[end_pos], lst[start_pos]
        start_pos += 1
        end_pos -= 1
    return lst

Time complexity - The time complexity of the code is O(n), where n is the difference between "end_pos" and "start_pos". Due to the swapping of two elements at each iteration, the while loop iterates n/2 times. Therefore, the time complexity of the while loop is O(n/2), which is equivalent to O(n).

Space complexity - The space complexity of the code is O(1) because the algorithm modifies the input list in-place without creating any new data structures. As a result, the function uses the same amount (constant) of space regardless of the size of the input list.

For more Practice: Solve these Related Problems:

Write a Python function that takes a list and an index, then returns a new list with the portion after that index reversed.
Write a Python function to reverse only the elements between two specified indices in a list.
Write a Python function to reverse the sublist starting from a given index to the end of the list in-place.
Write a Python function that reverses every segment of a list separated by a specific delimiter value.

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